Those interested in obtaining information about objects when using radar have relied primarily on two types of methods. One of them requires the object to have an active transmitter that transmits a signal in response to an interrogating radar signal. An example of this type system is commonly referred to as IFF (Identification Friend or Foe). The other method relies on the physical characteristics of the object to shape the reflected interrogating signal, from which the receiver derives information.
A more specific example of the first method is embodied in U.S. Pat. No. 3,270,338. This patent utilizes a radar transmitter that generates a pulse train. A "coded station" then receives the pulse train and, based on a predetermined code, transmits a pulse train which contains station dependent information. This patent is illustrative of an identification system where the object or "target" actively transmits a signal based on the interrogation signal.
Inherent disadvantages to the above method include the added costs and complexity of an active receiver/transmitter at the recipient station. Moreover, in hostile environments, transmitting an active signal facilitates location of the object's position by adversaries.
While relying on an active transmitter to convey information from interrogating signals has inherent disadvantages, employing the second method which uses no processing means and merely reflects the interrogating signal is also inadequate under certain circumstances. An example of this method can be seen in U.S. Pat. No. 3,247,514. This patent uses microwaves to identify railroad cars. Each railroad car is identified by a predetermined set of reflector elements. Depending upon the shape and configuration of the reflectors, information about the particular railroad car will be reflected back to the transmitter/receiver.
While this method does not have the added cost and complexity of an active transmitter, it requires physical alteration of the object or the installation of reflectors in order to reprogram or modify the reflected information. Moreover, the reflected signal may be weak and noisy thereby reducing range and sensitivity.
Another example of using an RF interrogating signal and returning a coded signal is shown in U.K. patent 1,433,140. A signal is received through a transducer in an acoustic surface wave delay line. Several transducers, each with a dipole antenna, are connected at spaced locations along the delay line. The outputs of the dipoles are selectively encoded by means of a mask having holes therethrough. Where a hole corresponds in position to a dipole, a delayed signal is transmitted. Thus a physical masking arrangement is employed to create a reflected code. The frequencies involved are in the 10,000 MHz range.
The need for positive yet reasonably secure identification of objects has become evident from events involving friendly forces being mistakenly attacked by other friendly forces, where visual identification was not conveniently possible before the tragic attack was made. Because of the ranges and speeds involved in current weaponry, the identification challenges have been accentuated. Because of positive identification limitations, the full capabilities of some armaments cannot be fully explained.
Although the IFF (as a general term) problem has existed throughout the history of warfare, the tempo of modern battle coupled with the range and lethality of modern weapons systems require a material solution to minimize the inadvertent destruction of friendly forces. Weather is not a factor in detecting a target, nor is range limited to line of sight. Positive identification of targets has lagged the ability to detect and destroy them.
Another factor which aggravates the IFF problem is non-linearity of the battlefield. Overlapping islands of conflict, increased weapons ranges and the confusion that occurs during rapid movement of tactical forces increase the need for passive and active IFF systems.